Axial motors, such as a submersible pump motor, have multiple rotors, each separated by a rotor bearing and retained on the motor's central shaft by retaining rings. These rotors are constructed primarily from copper while the shaft is primarily steel. As the motor runs, it generates heat and causes the components to expand. There is a substantial difference in the rate of thermal expansion between copper and steel, thus the rotors expand significantly more than the shaft. In some cases, this difference in expansion can result in the rotor stack being as much as half an inch longer than the shaft. If the rotor stack is constrained by the retaining rings and not allowed to expand relative to the shaft, the rotors expand into the bearings and prevent the bearings from rotating. The friction from the locked bearings causes tremendous heat and ultimate failure.
The prior art motors have loosely fit the rotor stacks on the central shaft, allowing a clearance between the rotor stack and the upper retaining ring. This provides room into which the rotors can expand and prevents the rotors from expanding into the bearings. However, the clearance introduces new problems. The rotors of the prior art can freely slide on the shaft and be damaged as they slam into each other during transport. Further, during start-up, the rotor stack can jump upwards on the shaft. As the rotor stack settles down, one or more rotor bearings can become misaligned and wedge in the stator bore. A wedged bearing may prevent the rotors beneath from expanding freely as the motor warms up, and as discussed above, prevent the bearings from rotating and cause a motor failure.
Therefore, there is a need for a motor that allows for the variance in thermal expansion of the rotor stacks and central shaft, yet protects the rotor stack from damage during transport and prevents rotor bearing misalignment and failure at start-up.